Timing. In the olden days when all media was printed, it took several days or weeks to produce a newspaper or magazine. If the press wanted to report on new products, they needed early access to have the reports as close to launch as possible.
These days, the timing is easier for online media but it still applies.
For many reasons - competition, need to brief retailers/partners - companies usually don't want details in the public domain before they're ready. Briefings under NDA give the media the information they need, and the company the protection they want.